Multi-hypothesis prediction

1. Video decoder for decoding a video from a data stream using motion compensated prediction for inter predicted blocks, comprising a hypothesis number control, configured to:

control a number of prediction hypotheses of the inter predicted blocks within a predetermined portion of the video to meet a predetermined criterion,

read from the data stream information on the number of prediction hypotheses for a currently processed inter predicted block dependent on a number of prediction hypotheses of previously processed inter predicted blocks within the predetermined portion and the predetermined criterion, and

based on the number of prediction hypotheses of the previously processed inter predicted blocks within the predetermined portion and the predetermined criterion, determine an allowed number interval, and, using a binarization of the allowed number interval, code the information into the data stream,

wherein the predetermined criterion indicates a target mean number of prediction hypotheses per inter predicted block or a maximum number of the hypotheses for the inter predicted blocks within the predetermined portion.

2. Video decoder according to claim 1 , wherein the hypothesis number control is configured to continuously update a mean number of prediction hypotheses per inter predicted block processed within the predetermined portion of the video and/or an accumulated number of prediction hypotheses of the inter predicted blocks within the predetermined portion, and determine the number of hypotheses for a current processed inter predicted block dependent on the predetermined criterion and the mean number and/or the accumulated number.

3. Video decoder according to claim 1 , configured so that the number of prediction hypotheses per inter predicted block is limited to an upper limit.

4. Video decoder according to claim 1 , wherein the predetermined portion is a collocated tile within a sequence of pictures, a slice, a picture, a group of pictures, a whole video sequence.

5. Video decoder according to claim 1 , configured to read from the data stream information on the number of prediction hypotheses for each inter predicted block.

6. Video decoder according to claim 1 , configured so that a maximum of the allowed number interval is the lower, the larger the number of prediction hypotheses of the previously processed inter predicted blocks within the predetermined portion is, and the binarization of the allowed number interval comprises a mean codeword length which is the lower, the lower the maximum of the allowed number interval is.

7. Method for decoding a video from a data stream using motion compensated prediction for inter predicted blocks, comprising a hypothesis number control, the method comprising:

controlling a number of prediction hypotheses of the inter predicted blocks within a predetermined portion of the video to meet a predetermined criterion,

reading from the data stream information on the number of prediction hypotheses for a currently processed inter predicted block dependent on a number of prediction hypotheses of previously processed inter predicted blocks within the predetermined portion and the predetermined criterion, and

based on the number of prediction hypotheses of the previously processed inter predicted blocks within the predetermined portion and the predetermined criterion, determining an allowed number interval, and, using a binarization of the allowed number interval, code the information into the data stream,

wherein the predetermined criterion indicates a target mean number of prediction hypotheses per inter predicted block or a maximum number of the hypotheses for the inter predicted blocks within the predetermined portion.

8. A non-transitory digital storage medium having a computer program stored thereon to perform a method for decoding a video from a data stream using motion compensated prediction for inter predicted blocks, comprising a hypothesis number control, the method comprising:

controlling a number of prediction hypotheses of the inter predicted blocks within a predetermined portion of the video to meet a predetermined criterion,

reading from the data stream information on the number of prediction hypotheses for a currently processed inter predicted block dependent on a number of prediction hypotheses of previously processed inter predicted blocks within the predetermined portion and the predetermined criterion, and

based on the number of prediction hypotheses of the previously processed inter predicted blocks within the predetermined portion and the predetermined criterion, determining an allowed number interval, and, using a binarization of the allowed number interval, code the information into the data stream,

wherein the predetermined criterion indicates a target mean number of prediction hypotheses per inter predicted block or a maximum number of the hypotheses for the inter predicted blocks within the predetermined portion,

when said computer program is run by a computer.

9. Video encoder for encoding a video into a data stream using motion compensated prediction for inter predicted blocks, comprising a hypothesis number control, configured to:

control a number of prediction hypotheses of the inter predicted blocks within a predetermined portion of the video to meet a predetermined criterion,

insert into the data stream information on the number of prediction hypotheses for each inter predicted block, and

insert into the data stream information on the number of prediction hypotheses for a currently processed inter predicted block dependent on a number of prediction hypotheses of previously processed inter predicted blocks within the predetermined portion and the predetermined criterion,

wherein the predetermined criterion indicates a target mean number of prediction hypotheses per inter predicted block or a maximum number of the hypotheses for the inter predicted blocks within the predetermined portion.